JP2017002333A - High strength steel sheet excellent in shape freezing property and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet excellent in shape freezing property and having tensile strength of 440 N/mmor more and proper yield point elongation and a manufacturing method therefor.SOLUTION: There is provided a high strength steel sheet excellent in shape freezing property, containing, by mass%, C: 0.03% to 0.35% (inclusive), Si: 0.01% to 2.00% (inclusive), Mn: 0.3% to 4.0% (inclusive), P: 0.001% to 0.100% (inclusive), S: 0.0005% to 0.050% (inclusive), N: 0.0005% to 0.010% (inclusive), Al: 0.01% to 2.00% (inclusive) and the balance Fe with inevitable impurities and having a metallic structure containing over 20% and 35% or less by area percentage of non-recrystallized ferrite phase and yield point elongation of 4.0% to 10.0% (inclusive).SELECTED DRAWING: Figure 1

Description

The present invention relates to a high strength steel sheet that is suitable as a pressed steel sheet for automobiles, has a suitably controlled yield point elongation, has a tensile strength of 440 N / mm ² or more, and has excellent shape freezing properties, and a method for producing the same. .

  In recent years, the weight reduction of automobile bodies as a measure for improving the fuel efficiency of automobiles and the need for passenger protection in the event of a collision have increased, and the strength of steel sheets for automobiles has been increased. The workability of steel sheets deteriorates with increasing strength. For this reason, development of high-strength steel sheets excellent in workability, such as DP (Dual Phase) steel and TRIP (Transformation Induced Plasticity) steel, has been promoted.

The impact absorbing member is required to increase the absorbed energy at the time of collision while suppressing the deformation amount from the viewpoint of occupant protection at the time of collision. In order to meet this requirement, as described in Non-Patent Document 1, it is effective to increase the yield stress of the steel sheet used as the material of the shock absorbing member.
On the other hand, by processing the impact absorbing member into a shape having a small bending radius R (mm), it is possible to improve the impact absorbing characteristics of the impact absorbing member. In order to obtain an impact-absorbing member having such a shape, it is necessary to use a steel plate excellent in shape freezing property.

Therefore, by using a steel plate having high strength and yield stress and excellent shape freezing property, an impact absorbing member having excellent impact absorbing characteristics can be obtained. Further, by using such a steel plate, a component having low residual stress and excellent fatigue characteristics can be obtained.
However, as described in Non-Patent Document 2, the yield stress and strength of the steel sheet and the shape freezing property have a very strong inverse correlation and are difficult to achieve at the same time.

  Until now, as described in Non-Patent Document 3, many techniques for improving the shape freezing property depend on a method for forming a steel sheet. Patent Document 1 proposes a technique for controlling the texture of a steel sheet as a technique for improving the shape freezing property. However, the technique described in Patent Document 1 has many restrictions on the manufacturing method in order to control the texture.

  On the other hand, Patent Document 2 describes a technique for utilizing non-recrystallized ferrite as a reinforcing structure by uniformly dispersing non-recrystallized ferrite containing many dislocations as a high-strength steel sheet having a high yield ratio. . Patent Document 2 describes a technique that achieves both high yield stress and workability by controlling the ratio of the lengths of recrystallized ferrite and non-recrystallized ferrite in the rolling direction.

Patent Document 3 reports a technique for improving local ductility and improving Young's modulus of a cold-rolled steel sheet by utilizing non-recrystallized ferrite that is softer than the hard second phase. Patent Document 4 introduces a technique for increasing Young's modulus and Rankford value (r value) by utilizing non-recrystallized ferrite.
However, the technique described in Patent Documents 2 to 4 is not a technique related to improvement of shape freezing property.

International Publication No. 00/006791 JP 2008-156680 A JP 2008-106352 A JP 2009-114523 A

Automobile Engineering Society Spring Academic Lecture Proceedings, 1973 P60. Press molding handbook 3rd edition Nikkan Kogyo Shimbun (2007) P346. Press molding handbook 3rd edition Nikkan Kogyo Shimbun (2007) P342.

This invention is made | formed in view of such a conventional situation, and provides the high strength steel plate excellent in the shape freezing property which has the tensile strength of 440 N / mm < ² > or more, and appropriate yield point elongation, and its manufacturing method. The task is to do.

The present inventors paid attention to non-recrystallized ferrite and conducted intensive studies on a measure for increasing the shape freezing property while maximally utilizing the hardness of the non-recrystallized ferrite.
As a result, by making the area ratio of the non-recrystallized ferrite phase contained in the metal structure more than 20% and 35% or less, the yield point elongation can be 4.0% or more and 10.0% or less, It was found that the shape freezing property can be enhanced without causing local bending constriction.

In addition, the present inventors have studied a method for producing a high-strength steel sheet having a metal structure that appropriately contains non-recrystallized ferrite. As a result, after pickling the hot-rolled steel sheet, the cold-rolled steel sheet obtained by performing cold rolling at a rolling rate of 15% or more was subjected to component composition, crystal grain size of the hot-rolled steel sheet, and cold rolling. It was found that it is important to perform annealing at a recrystallization temperature (recrystallization completion temperature) of −30 ° C. or lower and a recrystallization temperature of −60 ° C. or higher as determined by the rolling rate.
This invention is made | formed based on such knowledge, The summary is as follows.

(1) In mass%,
C: 0.03% or more, 0.35% or less,
Si: 0.01% or more, 2.00% or less,
Mn: 0.3% or more and 4.0% or less,
P: 0.001% or more, 0.100% or less,
S: 0.0005% or more, 0.050% or less,
N: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, containing 2.00% or less, the balance consists of Fe and inevitable impurities,
The metal structure contains an unrecrystallized ferrite phase with an area ratio of more than 20% and 35% or less,
A high-strength steel sheet excellent in shape freezing property, characterized by having a yield point elongation of 4.0% or more and 10.0% or less.

(2) The area ratio of 70% or more of the non-recrystallized ferrite phase is composed of non-recrystallized ferrite grains having a major axis length of 50 μm or less,
The high-strength steel sheet having excellent shape freezing property according to (1), wherein a connection ratio of the non-recrystallized ferrite grains is 0.50 or less.

(3) Furthermore, in mass%,
Cr: 0.05% or more, 3.0% or less,
Mo: 0.05% or more, 1.0% or less,
Ni: 0.05% or more, 3.0% or less,
Cu: The high strength steel plate excellent in shape freezing property as described in (1) or (2) characterized by containing 1 type or 2 types or more of 0.05% or more and 3.0% or less.

(4) Furthermore, in mass%,
Nb: 0.005% or more, 0.30% or less,
Ti: 0.005% or more, 0.30% or less,
V: 0.01% or more and 0.50% or less of one type or two types or more, (1) to (3) high strength steel plate excellent in shape freezing property .

(5) Furthermore, in mass%,
B: A high-strength steel sheet excellent in shape freezing property according to any one of (1) to (4), characterized by containing 0.0001% or more and 0.100% or less.

(6) Furthermore, in the steel by mass%,
Ca: 0.0005% or more, 0.010% or less,
Mg: 0.0005% or more, 0.010% or less,
Zr: 0.0005% or more, 0.010% or less,
REM: 0.0005% or more, 0.010% or less of 1 type or 2 types or more, The high strength steel plate excellent in shape freezing property according to any one of (1) to (5) .

(7) The high-strength steel sheet having excellent shape freezing property according to any one of (1) to (6), further comprising a hot-dip galvanized layer on the surface.

(8) (1) 15% or more after casting slab comprising the component composition according to any one of (3) to (6) is heated to 1100 ° C. or higher, hot-rolled, and pickled. After performing a cold rolling at a rolling rate of 1 ° C./sec after performing an annealing step of annealing the obtained cold-rolled steel sheet at a recrystallization temperature of −30 ° C. or lower and a recrystallization temperature of −60 ° C. or higher. As mentioned above, the manufacturing method of the high strength steel plate excellent in the shape freezing property characterized by performing the cooling process cooled to 480 degrees C or less with the average cooling rate of 200 degrees C / sec or less.

(9) In the annealing step, the shape is held at a temperature range of not more than the maximum temperature and not more than the maximum temperature of −30 ° C. for not less than 15 seconds and not more than 250 seconds. A method for producing a strength steel plate.
(10) The method for producing a high-strength steel sheet having excellent shape freezing property according to (8) or (9), wherein hot-dip galvanization is performed after the cooling step and alloying treatment is performed.

  According to the present invention, by including a non-recrystallized ferrite phase in a properly controlled area ratio, a high-strength steel sheet having an appropriate yield point elongation and high shape strength but excellent shape freezing properties is provided. it can. The high-strength steel sheet of the present invention is suitable as a material for an impact absorbing member that is required to have a collision absorbing characteristic, for example. Moreover, by using the high-strength steel sheet of the present invention, a component having low residual stress and excellent fatigue characteristics can be obtained. Further, by using the high-strength steel sheet of the present invention as a material for an automobile member such as an impact absorbing member, for example, it is possible to improve the fuel consumption by reducing the weight of the vehicle body and reduce the manufacturing cost by improving the shape freezing property. Therefore, the present invention has great industrial value.

It is the graph which showed the relationship between the area rate of a non-recrystallized ferrite phase, and yield point elongation. It is the graph which showed the relationship between yield stress and elongation.

Hereinafter, the high-strength steel sheet of the present invention and the manufacturing method thereof will be described in detail.
Non-recrystallized ferrite has long been recognized as a hard ferrite. For this reason, non-recrystallized ferrite has been used as a means for increasing the strength of a steel sheet at low cost.
The inventors of the present invention have made extensive studies on measures for improving the shape freezing property while maximally utilizing the hardness of non-recrystallized ferrite.

  As a result, it was found that the area ratio of the non-recrystallized ferrite phase contained in the metal structure had a correlation with the yield point elongation when having a specific component composition. For this reason, it has been found that the yield point elongation can be changed by controlling the area ratio of the non-recrystallized ferrite phase.

  Since the non-recrystallized ferrite phase is a hard phase, the yield stress of the steel sheet containing it is likely to be high. In addition, since the non-recrystallized ferrite phase has a higher dislocation density in the grains than the recrystallized ferrite phase, it contains many immobilized dislocations that improve the yield point elongation of the steel sheet. In particular, in the non-recrystallized ferrite phase of the low carbon steel having a carbon content of 0.30% or less, there are many immobilized dislocations. From these facts, by controlling the area ratio of the non-recrystallized ferrite phase, it is possible to obtain a steel plate in which the immobilized dislocations are present appropriately, and the yield point elongation can be controlled within an appropriate range.

  The present inventors have obtained an excellent shape freezing property and a yield point of 4.0% or more and 10.0% or less that is less likely to cause local bending necking during processing due to excessive elongation of the yield point. The range of the area ratio of the non-recrystallized ferrite phase from which elongation was obtained was examined. As a result, it was found that when the area ratio of the non-recrystallized ferrite phase is more than 20% and 35% or less, an appropriate yield point elongation of 4.0% or more and 10.0% or less can be obtained.

  Furthermore, the present inventors have repeatedly investigated and controlled the length of the major axis and the connection ratio of the non-recrystallized ferrite grains contained in the non-recrystallized ferrite phase to improve the uniformity of local deformation sites in the metal structure. Thus, it was found that the shape freezing property is further improved. When the area ratio of 70% or more of the non-recrystallized ferrite phase is composed of non-recrystallized ferrite grains having a major axis length of 50 μm or less, and the connection ratio of the non-recrystallized ferrite grains is 0.50 or less. The yield stress can be increased while suppressing the deterioration of elongation, and the shape freezing property is higher than that of the steel plate whose yield point elongation is the same and the length of the major axis of unrecrystallized ferrite grains and the connection ratio are not within the above ranges. Was confirmed to improve.

  The reason why the above effect can be obtained by setting the length of the major axis and the coupling ratio of the unrecrystallized ferrite grains in the above range is not clear, but a relatively large amount of immobilized dislocations in the unrecrystallized ferrite grains are present. It is presumed that the contribution to the shape freezing property due to the yield point elongation became significant due to the uniform dispersion in the steel sheet.

The high-strength steel sheet of the present invention has improved shape freezing properties by controlling the area ratio of the non-recrystallized ferrite phase, the length of the major axis of the non-recrystallized ferrite grains, and the connection ratio.
In the method for producing a high-strength steel sheet according to the present invention, the rolling rate in cold rolling is controlled, and the maximum temperature in the annealing process is determined by the component composition, the crystal grain size of the hot rolled steel sheet, and the rolling rate in cold rolling. Strictly control according to the crystal temperature.

The thickness of the high-strength steel plate obtained using the high-strength steel plate of the present invention and the manufacturing method thereof is not particularly limited, but is preferably a thin steel plate of 0.1 to 11.0 mm. A high-strength thin steel sheet having a thickness in the above range can be preferably used as a material for automobile members produced by press working such as an impact absorbing member. In addition, a high-strength thin steel sheet having a thickness in the above range can be easily produced using a thin sheet production line.
The high strength steel sheet of the present invention preferably has a tensile strength of 440 N / mm ² or more. A high-strength steel plate having a tensile strength in the above range is particularly suitable as a material for automobile members such as impact absorbing members.

Hereinafter, each component of the present invention will be described in detail.
First, the reasons for limiting the steel plate material will be described.
The high-strength steel sheet of the present invention has a yield point elongation of 4.0% or more and 10.0% or less. The reason why the shape freezing property is improved by setting the yield point elongation to the above range is not clear, but the yield point phenomenon promotes the localization of strain, so if the yield point elongation is sufficiently large, the yield phenomenon occurs across the entire plate thickness. Is presumed to occur.

  As the yield point elongation of the steel plate is larger, the effect of improving the shape freezing property is exhibited even under a condition where R / t (R: bending radius, t plate thickness) is small. In order to obtain a sufficient shape freezing property in a steel sheet for automobiles, the elongation at yield point needs to be 4.0% or more. On the other hand, if the yield point elongation is too large, work hardening hardly occurs after the yield point elongation. For this reason, it becomes easy to generate | occur | produce the local bending constriction which causes a bending crack at the time of a process. Therefore, the upper limit of the yield point elongation is 10.0%. In particular, in a high-strength steel sheet, it is desirable that the yield point elongation is 8.0% or less in order to prevent the occurrence of bending necking.

  Moreover, when the high-strength steel plate of this invention is a steel plate used as a material of an impact-absorbing member, it is preferable that YR (yield stress / tensile strength) is 0.75 or more. The higher the YR, the higher the stress in the low strain region with the same tensile strength. For example, when an impact absorbing member is formed using this, excellent shock absorbing characteristics can be obtained.

Next, the reasons for limiting the component composition of the present invention will be described. Hereinafter, unless otherwise specified,% means mass%.
C contributes to the strengthening of steel. The C content is required to be 0.03% or more in order to ensure strength, and is preferably 0.04% or more. On the other hand, it is an element that affects the workability of steel, and when the C content increases, the workability deteriorates. When the C content exceeds 0.35%, a large amount of bainite, pearlite, and martensite are generated as the second phase, and these influence each other with the non-recrystallized ferrite, thereby reducing the elongation. On the other hand, if the C content exceeds 0.35%, the hole-expanding property is remarkably lowered due to generation of harmful carbides (cementite). For this reason, C content shall be 0.35% or less. However, when particularly high hole expansibility is required, the C content is desirably 0.10% or less.

  Si increases the strength of the steel by solid solution strengthening, and has a small decrease in ductility. Therefore, Si is an effective element for increasing the strength of steel. Si also suppresses the generation of harmful carbides and is effective in improving workability. The action of suppressing the formation of carbides can be replaced by Al. From the above, Si is added by 0.01% or more. In particular, when not adding 0.10% or more of Al, it is desirable to add 0.30% or more of Si. However, when there is too much Si content, chemical conversion processability will fall and spot weldability will also deteriorate. For this reason, Si content makes 2.00% an upper limit.

  Al is an element effective for suppressing the formation of harmful carbides and improving the elongation, like Si described above. Conventionally, Al is an element necessary for deoxidation and has been added in an amount of about 0.01 to 0.07%. As a result of intensive studies, the present inventors have found that chemical conversion property can be improved without degrading ductility by adding a large amount of Al in a low Si system. However, when there is too much Al content, the effect of ductility improvement will be saturated, and chemical conversion property and spot weldability will also deteriorate. For this reason, it is desirable that the upper limit of the Al content is 2.00%, and the upper limit is 1.00% particularly under severe conditions of chemical conversion treatment. For sufficient deoxidation, 0.01% or more of Al needs to be added.

  Mn is an element necessary for ensuring strength. Mn also contributes to the formation of unrecrystallized ferrite by delaying recrystallization. In order to acquire this effect, it is necessary to make Mn content 0.3% or more. However, when Mn is added in a large amount, microsegregation and macrosegregation are likely to occur and the elongation is deteriorated. Therefore, the upper limit of the Mn content is 4.0%.

  P is an element that increases the strength of the steel sheet, and is an element that improves corrosion resistance when added simultaneously with Cu. However, when the P content is high, deterioration of weldability, workability, and toughness is caused. Accordingly, the P content is 0.100% or less. In particular, when the corrosion resistance is not a problem, it is desirable that the P content is 0.030% or less with emphasis on workability. However, since it takes a cost to reduce the P content, the lower limit is set to 0.001% from the viewpoint of de-P cost.

  S forms sulfides such as MnS and becomes a starting point of cracking, and reduces the hole expandability of workability. Therefore, the S content needs to be 0.050% or less. However, desulfurization cost becomes high in order to make S content less than 0.0005%. For this reason, S content shall be 0.0005% or more.

  N deteriorates workability. Further, when Ti and / or Nb is added together with N, the effective amount of Ti and Nb is reduced by the generation of TiN and NbN, and the generated nitride reduces the yield point elongation and the hole expandability. . For this reason, it is better that the N content is small. From the above constraints, the N content is set to 0.010% or less. From the viewpoint of removing N cost, the lower limit of the N content is set to 0.0005%.

The high-strength steel sheet of the present invention may further contain the following elements as necessary.
Ti, Nb, and V all form carbides and are effective in increasing the strength. In order to effectively exhibit this effect, it is necessary to add one or more of Ti, Nb, and V. Ti, Nb, and V delay recrystallization and contribute to the formation of non-recrystallized ferrite. In order to obtain these effects, it is necessary to add 0.005% or more of Ti, 0.005% or more of Nb, and 0.01% or more of V. However, when Ti, Nb, and V are excessively added, the yield point elongation deteriorates due to precipitation strengthening. Therefore, the Ti content is 0.30% or less, the Nb content is 0.30% or less, and the V content is 0.50% or less.

  Ca, Mg, Zr, and REM control the shape of sulfide inclusions and are effective in improving workability. In order to effectively exhibit this effect, it is necessary to add one or more of Ca, Mg, Zr, and REM. At this time, 0.0005% or more of each element needs to be added. On the other hand, a large amount of addition deteriorates the cleanliness of the steel, leading to a decrease in elongation. From this, when it contains 1 type, or 2 or more types of Ca, Mg, Zr, and REM, the upper limit of each addition amount shall be 0.010%.

Cu is an element that improves the corrosion resistance when combined with P. In order to obtain this effect, it is desirable to add 0.05% or more of Cu. However, the addition of a large amount of Cu increases the hardenability and decreases the ductility. For this reason, the upper limit of Cu content is set to 3.0%.
Ni is an element that suppresses hot cracking when Cu is added. In order to obtain this effect, it is desirable to add 0.05% or more of Ni. However, the addition of a large amount of Ni increases the hardenability and lowers the ductility like Cu. For this reason, the upper limit of Ni content is set to 3.0%.

  Mo is an element effective for suppressing the formation of cementite, contributing to strength, and improving hole expansibility. In order to acquire this effect, it is necessary to add 0.05% or more of Mo. However, since Mo is also an element that enhances hardenability, if Mo is added excessively, ductility decreases. For this reason, the upper limit of the Mo content is set to 1.0%.

  Cr, like V, forms carbides and is an element that contributes to securing strength. In order to obtain this effect, it is necessary to add 0.05% or more of Cr. However, since Cr is an element that enhances the hardenability, elongation is reduced when added in a large amount. Therefore, the upper limit of Cr content is set to 3.0%.

  B is an element that contributes to strength in the same manner as Mn. In order to obtain this effect, the B content needs to be 0.0001% or more. However, since B is also an element that enhances the hardenability, the ductility decreases when added in a large amount. For this reason, the upper limit of the B content is set to 0.100%.

Next, the metal structure of the high strength steel plate of the present invention will be described.
In the present invention, one of the most important features is the area ratio of the non-recrystallized ferrite phase. In order to increase the yield point elongation, it is preferable to contain a large amount of unrecrystallized ferrite phase. However, when there are too many non-recrystallized ferrite phases, the yield point elongation becomes too large and local bending necking tends to occur during processing. For this reason, the area ratio of the non-recrystallized ferrite phase needs to be more than 20% and not more than 35%. In particular, when the steel sheet is used under severe processing conditions, the area ratio of the non-recrystallized ferrite phase is desirably 30% or less.

"Calculation method of area ratio of non-recrystallized ferrite phase"
The area ratio of the non-recrystallized ferrite phase contained in the high-strength steel sheet of the present invention is determined by the following method.
A sample is taken with a plate thickness cross section parallel to the rolling direction as an observation surface, and the observation surface is polished, nital etched, and if necessary, repeller etched, then observed with an optical microscope and photographed. By analyzing the image of the obtained micrograph, the ferrite phase and other than the ferrite phase are distinguished, and the area ratio of the ferrite phase is calculated. In addition, the area ratio of the ferrite phase is calculated by calculating the area ratio of the ferrite phase by analyzing one image of 10 or more micrographs having different fields of view, with one field of view of the micrograph being 200 μm in length and 200 μm in width. Calculate by averaging.

  Further, the steel sheet is reduced to a predetermined plate thickness by mechanical polishing or the like, and the distortion is removed by electrolytic polishing or the like, and at the same time, a sample is prepared so that the 1/4 thickness surface becomes the measurement surface. Crystal orientation measurement data in an electron backscattering analysis image (referred to as EBSP) is obtained for the measurement surface of the prepared sample. EBSP measures 5 points or more in each crystal grain of the sample. Crystal orientation measurement data obtained from each EBSP measurement result is output as a pixel.

  Next, the obtained crystal orientation measurement data is analyzed by the Kernel Average Misoration (KAM) method, the unrecrystallized ferrite contained in the ferrite phase is discriminated, and the area ratio of the unrecrystallized ferrite in the ferrite phase is calculated. In the KAM method, the crystal orientation difference between adjacent pixels (measurement points) can be quantitatively shown. In the present invention, grains having an average crystal orientation difference of 1 ° or more from adjacent measurement points are defined as non-recrystallized ferrite.

  Next, the area ratio of unrecrystallized ferrite in the metal structure is calculated using the area ratio of the ferrite phase in the metal structure of the high-strength steel sheet and the area ratio of unrecrystallized ferrite contained in the ferrite phase. .

  Further, the length of the major axis and the connection rate of the non-recrystallized ferrite grains indicating the dispersion state of the non-recrystallized ferrite phase affect the shape freezing property. In order to contribute to the shape freezing property, the area ratio of 70% or more of the non-recrystallized ferrite phase is made of non-recrystallized ferrite grains having a major axis length of 50 μm or less. The connection rate is preferably 0.50 or less. In order to further improve the shape freezeability of the steel sheet, the area ratio of non-recrystallized ferrite grains having a major axis length of 50 μm or less in the non-recrystallized ferrite phase is more than 80%. Preferably, the connection rate of non-recrystallized ferrite grains is more preferably 0.40 or less.

  Note that the area ratio of 70% or more of the non-recrystallized ferrite phase is the non-recrystallized ferrite grains having a major axis length of 50 μm or less, and for each of the two or more non-recrystallized ferrite phases, The length of the major axis of about 100 non-recrystallized ferrite grains is measured by the method shown in FIG. 1, and the ratio of the unrecrystallized ferrite grains whose major axis length is 50 μm or less with respect to the number of measurements {(unrecrystallized less than 50 μm The number of ferrite grains / the number of non-recrystallized ferrite grains measured) × 100} is calculated, which means that the average value is 70% or more.

"Measuring method of length of major axis of non-recrystallized ferrite grains"
The length of the major axis of the non-recrystallized ferrite grains is measured using the observation result of the non-recrystallized ferrite grains obtained from the crystal orientation analysis in a two-dimensional optical microscope or EBSP. However, when two or more non-recrystallized ferrite grains are present in contact with each other and the following conditions are satisfied, a part or all of the lump of two or more non-recrystallized ferrite grains is combined into one non-recrystallized ferrite. Treat as grains. That is, the length of the interface between the non-recrystallized ferrite grains is 1/5 or more of the length of the interface with the shorter interface length among the non-recrystallized ferrite grains existing in contact with each other. Is the case. In this case, the length of the major axis of the non-recrystallized ferrite grains is defined as the length of the major axis of the mass of the non-recrystallized ferrite grains.

"Calculation method of the connection ratio of non-recrystallized ferrite grains"
The connection ratio of non-recrystallized ferrite grains is the number B of the total number A of crystal grains adjacent to non-recrystallized ferrite grains, where the crystal grains adjacent to non-recrystallized ferrite grains are other non-recrystallized ferrite grains. It is a ratio (B / A). The connection ratio of non-recrystallized ferrite grains was calculated by calculating the connection ratio of about 100 non-recrystallized ferrite grains for each of the two or more non-recrystallized ferrite phases and averaging them.

  The total number A of crystal grains adjacent to the non-recrystallized ferrite grains and the number B in which the adjacent crystal grains are other non-recrystallized ferrite grains are obtained from the crystal orientation analysis in a two-dimensional optical microscope or EBSP. Measured using observation results of ferrite grains. However, when two or more non-recrystallized ferrite grains are present in contact with each other and the following conditions are satisfied, a part or all of the lump of two or more non-recrystallized ferrite grains is combined into one non-recrystallized ferrite. Treat as grains. That is, the length of the interface between the non-recrystallized ferrite grains is 1/5 or more of the length of the interface with the shorter interface length among the non-recrystallized ferrite grains existing in contact with each other. Is the case.

  The present invention is an invention that makes the best use of the non-recrystallized ferrite phase, and in the metal structure of the high-strength steel sheet of the present invention, there are no particular restrictions on the phases other than the non-recrystallized ferrite phase. .

  The high-strength steel sheet of the present invention may have a hot dip galvanized layer on the surface. Corrosion resistance is improved by providing a galvanized layer on the surface. The hot dip galvanized layer preferably contains Zn and Al and has a Fe content limited to less than 13%. When the Fe content in the hot dip galvanized layer is less than 13%, the plating adhesion, formability, and hole expandability are excellent. When the Fe content is 13% or more, the adhesiveness of the hot dip galvanized layer itself is impaired. For this reason, when processing a steel plate, the hot dip galvanized layer breaks and falls off and adheres to the mold, which causes wrinkles.

The hot dip galvanized layer may be alloyed. In the alloyed hot-dip galvanized layer, Fe is taken into the hot-dip galvanized layer by the alloying treatment, so that excellent spot weldability and paintability are obtained. In the alloyed hot-dip galvanized layer, the Fe content is preferably 7% or more. If the Fe content is less than 7%, the effect of improving the spot weldability by performing the alloying treatment may be insufficient.
In the case of an unalloyed hot dip galvanized layer, if the Fe content is less than 13%, even if it is less than 7%, the effect of having the hot dip galvanized layer is not affected, and is 0%. Also good.

The plating adhesion amount is not particularly limited, but is preferably 5 g / m ² or more in terms of one-side adhesion amount from the viewpoint of corrosion resistance.
In the high-strength steel sheet of the present invention, upper plating may be performed on the hot dip galvanized layer for the purpose of improving paintability and weldability. In the high-strength steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment and the like may be performed on the hot dip galvanized layer.

Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
In order to manufacture the high-strength steel sheet of the present invention, first, a cast slab having any one of the above component compositions is prepared. Next, the cast slab is directly or once cooled, and then heated to 1100 ° C. or higher, and hot rolling is performed.

In this embodiment, before performing hot rolling, heat treatment is performed to heat to 1100 ° C. or higher in order to homogenize the cast slab and dissolve the carbonitride. The heat treatment of the cast slab may be performed directly on the cast slab as it is at a high temperature after casting the cast slab, or may be performed by reheating the cast slab once cooled after casting.
When the heat treatment temperature of the cast slab is less than 1100 ° C., the homogenization of the cast slab and the dissolution of carbonitride are insufficient, resulting in a decrease in strength and workability. On the other hand, when the heat treatment temperature of the cast slab exceeds 1300 ° C., the manufacturing cost increases and the productivity decreases. On the other hand, if the heat treatment temperature exceeds 1300 ° C., the initial austenite grain size becomes large, and eventually it becomes easy to become mixed grains, which may reduce ductility. Therefore, the heat treatment temperature of the cast slab needs to be 1100 ° C. or higher, and is preferably less than 1300 ° C.

As for hot rolling, it is desirable to perform finish rolling at a temperature of ₃ or more points of Ar. If the finishing temperature of the hot rolling is lower than the Ar ₃ point, cracking in the cold rolling is induced, and there is a concern about deterioration of the material.
The Ar ₃ transformation temperature is the content of C, Si, P, Al, Mn, Mo, Cu, Cr, and Ni, expressed in mass%, respectively (% C) (% Si) (% P) (% Al) ( What is necessary is just to calculate by the following formula | equation using% Mn) (% Mo) (% Cu) (% Cr) (% Ni). In addition, Mo, Cu, Cr, and Ni, which are elements that are selectively added, are calculated as 0 when the content is about an impurity.
Ar ₃ = 901-325 × (% C) + 33 × (% Si) + 287 × (% P)
+ 40 × (% Al) −92 × (% Mn +% Mo +% Cu) −46 × (% Cr +% Ni)

  The hot-rolled steel sheet obtained after hot rolling is subjected to cold rolling after removing the scale layer by pickling. Cold rolling is performed at a rolling rate of 15% or more. When the rolling rate in cold rolling is less than 15%, recrystallization nuclei are unlikely to form, and grain growth starts due to coarsening of recovered grains. For this reason, recrystallization becomes insufficient, and it becomes difficult to obtain a metal structure in which the area ratio of non-recrystallized ferrite is 35% or less. The rolling ratio in cold rolling is preferably 25% or more in order to reduce the area ratio of non-recrystallized ferrite and prevent bending cracks in the steel sheet during processing due to excessive yield point elongation. . In addition, the rolling rate in cold rolling is preferably 80% or less in order to prevent deterioration of the steel sheet shape due to a high rolling load.

Next, an annealing process is performed in which the obtained cold-rolled steel sheet is annealed at a recrystallization temperature of −30 ° C. or lower and a recrystallization temperature of −60 ° C. or higher.
The annealing process is the most important process in forming the metal structure of the high-strength steel sheet of the present invention. The maximum reached temperature (maximum temperature) in the annealing process is controlled with respect to the recrystallization temperature. That is, it is necessary to set the maximum temperature to be a recrystallization temperature of −30 ° C. or lower and a recrystallization temperature of −60 ° C. or higher. When the maximum reached temperature exceeds the recrystallization temperature of −30 ° C., it becomes difficult to sufficiently leave the non-recrystallized ferrite phase. The maximum reached temperature is preferably −35 ° C. or less of the recrystallization temperature in order to easily secure the area ratio of unrecrystallized ferrite. On the other hand, if the maximum attainable temperature is less than the recrystallization temperature −60 ° C., the unrecrystallized ferrite phase remains too much and the yield point elongation becomes too large. The maximum temperature reached is preferably −55 ° C. or higher of the recrystallization temperature.

The recrystallization temperature is determined in advance by the following method depending on the component composition, the crystal grain size of the hot-rolled steel sheet, and the rolling rate in cold rolling, which are the main factors that change the recrystallization temperature.
"Calculation method of recrystallization temperature"
A cold-rolled steel sheet (as cold-rolled material) prepared with a predetermined component composition, crystal grain size of hot-rolled steel sheet, and rolling rate in cold rolling is heated with a dilatometer at a rate of temperature increase of 10 ° C./s. When it reaches the temperature reached, it is cooled to obtain a test specimen. In this embodiment, the ultimate temperature is changed at a pitch of 10 ° C. or less to create a plurality of test bodies having different ultimate temperatures. The area ratio of the non-recrystallized ferrite phase of each test specimen obtained is examined using the above-described “calculation method of the area ratio of the non-recrystallized ferrite phase”. And the highest temperature of the ultimate temperature of the test body in which the area ratio of the recrystallized ferrite phase was 98% or more is defined as the recrystallization temperature.

When determining the recrystallization temperature, accuracy is confirmed, and a physical model corresponding to the area ratio of the non-recrystallized ferrite phase of the steel sheet is used to select one of the plurality of test bodies. The area ratio of part or all of the unrecrystallized ferrite phase may be calculated, or the recrystallization temperature may be calculated.
Further, based on the area ratio of the non-recrystallized ferrite phase of a plurality of specimens, any one of the conditions (for example, the component composition) of the component composition, the crystal grain size of the hot-rolled steel sheet, and the rolling rate in cold rolling A map showing the relationship with the crystal temperature may be created and used to determine the recrystallization temperature, or an empirical formula between one or more of the above conditions and the recrystallization temperature may be created and used. Thus, the recrystallization temperature may be determined.

  In the present embodiment, in the annealing process, it is preferable to hold the temperature within a temperature range of not more than the maximum temperature and not less than the maximum temperature −30 ° C. for not less than 15 seconds and not more than 250 seconds. By holding for 15 seconds or more in the temperature range of the maximum temperature or lower and the maximum temperature of −30 ° C. or higher, the length of the major axis of the non-recrystallized ferrite grains is reduced and the connection rate is reduced. On the other hand, even if the holding time in the temperature range of the maximum temperature or lower and the maximum temperature of −30 ° C. or higher exceeds 250 seconds, the effect of reducing the length of the major axis of the non-recrystallized ferrite grains and the connection rate is saturated Since productivity falls and manufacturing cost increases, it is not preferable.

In this embodiment, after performing an annealing process, the cooling process which cools to 480 degrees C or less with the average cooling rate of 1 degree C / sec or more and 200 degrees C / sec or less is performed.
In the cooling step, it is necessary to cool at an average rate of 1 ° C./second or more in order to suppress material deterioration due to a change in structure during cooling. Moreover, even if the average cooling rate exceeds 200 ° C./second, the characteristics of the high-strength steel plate do not change greatly, and the accuracy of the cooling stop temperature is lowered and the cooling cost is increased. For this reason, the upper limit of an average cooling rate shall be 200 degrees C / sec.
In the cooling step, the cooling stop temperature is set to 480 ° C. or lower. When the cooling stop temperature exceeds 480 ° C., the yield point elongation of the high-strength steel sheet is significantly deteriorated due to the formation of pearlite.

In the present embodiment, hot dip galvanization may be performed after the cooling step. Thereby, a high-strength steel sheet having a hot-dip galvanized layer on the surface is obtained.
Furthermore, in this embodiment, after performing hot dip galvanization, you may perform an alloying process. When the alloying treatment is performed, it is preferably performed at a temperature of 600 ° C. or lower. When the temperature of the alloying treatment is 600 ° C. or less, it is preferable because the metal structure of the steel sheet after the cooling step can be suppressed from being changed by performing the alloying treatment.

Next, this invention is demonstrated based on an Example.
Steels having the composition shown in Table 1 were melted and cast slabs by a continuous casting method according to a conventional method.
In Table 1, as for the steel of code | symbol AL, a component composition has satisfy | filled this invention. Steel with symbol a contains C and Ca, steel with b contains Mn and P, steel with c contains Nb, steel with d contains C, steel with e contains Si and S The amount of f steel is N and Ti content is outside the scope of the present invention.

In Tables 2 and 3, the letters of steel represent the types of steel shown in Table 1, and the numerals represent the numbers of the examples. For example, “A1” means the first example using the steel A in Table 1.
In Tables 1 to 3, numerical values outside the scope of the present invention are underlined.

The cast slab having the component composition shown in Table 1 was heated, hot-rolled, pickled, cold-rolled, and subjected to an annealing process and a cooling process to obtain a steel sheet having a thickness of 1.4 mm.
Table 2 shows the casting slab heating temperature, Ar ₃ , hot rolling finishing temperature, cold rolling rolling rate, recrystallization temperature in annealing process—maximum temperature reached (maximum temperature), holding time at maximum temperature, cooling process The average cooling rate and cooling stop temperature are shown.

About a part of the obtained steel sheet, it is subjected to hot dip galvanizing according to a conventional method, and part of the hot dip galvanized part is subjected to alloying treatment at a temperature of 600 ° C. or less after being immersed in hot dip galvanizing, A specimen was used.
Moreover, about the steel plate which did not give hot dip galvanization, it used as it was as a specimen.
Table 2 shows the presence or absence of a hot-dip galvanized layer and the presence or absence of an alloying treatment.

Each specimen was evaluated for mechanical properties (yield stress, tensile strength, elongation, yield point elongation) in accordance with JIS Z2241. The results are shown in Table 3.
Further, for the specimen, using the above-described method, the area ratio of the non-recrystallized ferrite phase, the area ratio of the non-recrystallized ferrite grains having a major axis length of 50 μm or less in the non-recrystallized ferrite phase, The connection rate of the crystal ferrite grains was examined. The results are shown in Table 3.

About each specimen, shape freezing property was evaluated by the method shown below.
A 90-degree bending test was performed in accordance with the U-bending test method described on pages 131 to 133 of "Press Forming Difficulty Handbook 3rd Edition" (Nikkan Kogyo Shimbun Co., Ltd.) edited by the Research Committee for Thin Steel Sheet Forming Technology.
The value obtained by subtracting 90 degrees from the opening angle (spring back amount) is the spring back amount described in Patent Document 1 as the spring back amount of general steel (<3.66 × TS (tensile strength) +3.25). ) Were evaluated as “◯”, and those exceeding this were determined as “X”.
In addition, when the specimen used for the evaluation of shape freezing property was processed, “*” was given when bending necking occurred, and “−” was given when shape freezing property could not be evaluated due to insufficient strength. .
The results are shown in Table 3.

FIG. 1 is a graph showing the relationship between the area ratio of the non-recrystallized ferrite phase and the yield point elongation. The results of Invention Steel 1 and Invention Steel 2 shown in Table 3 and the component composition of the comparative steels of the present invention are shown in FIG. The result of satisfying the above is shown.
As shown in FIG. 1, when the component composition satisfies the present invention, there is a correlation between the area ratio of the non-recrystallized ferrite phase and the yield point elongation.

  In addition, as shown in FIG. 1, a specimen of an inventive steel that satisfies claim 1 of the present invention, in which the area ratio of the non-recrystallized ferrite phase is more than 20% and not more than 35% (invention steel 1 and Invention steel 2) had a yield point elongation of 4.0% or more and 10.0% or less. Further, as shown in Table 3, these inventive steel specimens had a small amount of springback and an evaluation of shape freezing property was good. In addition, a specimen of a comparative steel (a part of the comparative steel in the remarks in Table 3) in which the component composition satisfies the present invention and the area ratio of the non-recrystallized ferrite phase is outside the scope of the present invention has a yield point elongation. It was out of the scope of the invention, and as shown in Table 3, the amount of spring back was large and the evaluation of shape freezing property was x.

FIG. 2 is a graph showing the relationship between yield stress and elongation, and shows the measurement results of the yield stress and elongation of the steel shown in Table 3.
As shown in FIG. 2, the specimen of the invention steel satisfying claim 1 and claim 2 of the present invention (invention steel 1 in the remarks of FIG. 2 and Table 3) satisfies claim 1 of the present invention, and claims. It was confirmed that a high elongation was obtained with the specimen having the same yield stress compared to the specimen of invention steel not satisfying 2 (invention steel 2 in the remarks of FIG. 2 and Table 3).

Further, as shown in Table 3, the specimens satisfying claim 1 of the present invention (Invention Steel 1 and Invention Steel 2 in the remarks of Table 3) all have an evaluation of shape freezing property and have sufficient strength. Had.
On the other hand, among specimens not satisfying claim 1 of the present invention (comparative steel in the remarks in Table 3), specimens other than steel d1 had a shape freezing evaluation of x or *. Further, the strength of the steel d1 was insufficient.

Since steel A1 had a low rolling ratio of cold rolling, the area ratio of the non-recrystallized ferrite phase was increased. As a result, the yield point elongation became too large and bending necking occurred.
Since steel B1 had a low average cooling rate in the cooling step, the metal structure changed during cooling, and the area ratio of the non-recrystallized ferrite phase was reduced. As a result, the yield point elongation was insufficient, and the evaluation of shape freezing property was x.

Since the maximum temperature of steel E1 was too high, the area ratio of the non-recrystallized ferrite phase was reduced, yield point elongation was insufficient, and the evaluation of shape freezing property was x.
Since the maximum temperature of steel E5 was too low, the area ratio of the non-recrystallized ferrite phase was increased, the elongation at yield point was excessively increased, and bending necking occurred.

Steel G1 had a cooling stop temperature that was too high, so that the area ratio of the non-recrystallized ferrite phase was small, yield point elongation was insufficient, and the evaluation of shape freezing property was x.
Steel L2 was low in elongation due to the too low heating temperature of the cast slab and lacked in yield point elongation, so that work hardening did not occur and bending necking occurred.

Steel a1 has an excessive content of C and Ca, and therefore has low elongation and lack of yield point elongation. Therefore, work hardening does not occur and bending constriction occurs.
Steel b1 has an excessive content of Mn and P, and therefore has low elongation, and work hardening does not occur and bending constriction occurs.
In steel c1, since the Nb content is excessive, the correlation between the area ratio of the unrecrystallized ferrite phase and the yield point elongation cannot be obtained, the area ratio of the unrecrystallized ferrite phase is large, and the yield point elongation ( Compared with YP-El), the elongation was insufficient and the yield point elongation was insufficient, and the evaluation of the shape freezing property could not be performed.

Steel d1 had insufficient yield strength and tensile strength because of insufficient C content.
Steel e1 had an excessive content of Si and S, so that the yield point elongation was insufficient, and the evaluation of shape freezing property was x.
In steel f1, since the contents of N and Ti are excessive, there is no correlation between the area ratio of the non-recrystallized ferrite phase and the yield point elongation, and the area ratio of the non-recrystallized ferrite phase is large. The elongation was insufficient, and the evaluation of the shape freezing property was x.

Claims (10)

% By mass
C: 0.03% or more, 0.35% or less,
Si: 0.01% or more, 2.00% or less,
Mn: 0.3% or more and 4.0% or less,
P: 0.001% or more, 0.100% or less,
S: 0.0005% or more, 0.050% or less,
N: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, containing 2.00% or less, the balance consists of Fe and inevitable impurities,
The metal structure contains an unrecrystallized ferrite phase with an area ratio of more than 20% and 35% or less,
A high-strength steel sheet excellent in shape freezing property, characterized by having a yield point elongation of 4.0% or more and 10.0% or less. The area ratio of 70% or more of the non-recrystallized ferrite phase is composed of non-recrystallized ferrite grains having a major axis length of 50 μm or less,
The high-strength steel sheet having excellent shape freezing property according to claim 1, wherein a connection ratio of the non-recrystallized ferrite grains is 0.50 or less. Furthermore, in mass%,
Cr: 0.05% or more, 3.0% or less,
Mo: 0.05% or more, 1.0% or less,
Ni: 0.05% or more, 3.0% or less,
The high-strength steel sheet excellent in shape freezing property according to claim 1 or 2, characterized by containing one or more of Cu: 0.05% or more and 3.0% or less. Furthermore, in mass%,
Nb: 0.005% or more, 0.30% or less,
Ti: 0.005% or more, 0.30% or less,
V: Highly excellent in shape freezing property according to any one of claims 1 to 3, characterized by containing one or more of 0.01% or more and 0.50% or less. Strength steel plate. Furthermore, in mass%,
B: 0.0001% or more and 0.100% or less is contained, The high strength steel plate excellent in shape freezing property of any one of Claims 1-4 characterized by the above-mentioned. Furthermore, in steel,
Ca: 0.0005% or more, 0.010% or less,
Mg: 0.0005% or more, 0.010% or less,
Zr: 0.0005% or more, 0.010% or less,
REM: 0.0005% or more, 0.010% or less of 1 type or 2 types or more is contained, The shape excellent in shape freezing property of any one of Claims 1-5 characterized by the above-mentioned Strength steel plate.   Furthermore, it has a hot-dip galvanized layer on the surface, The high strength steel plate excellent in shape freezing property of any one of Claims 1-6 characterized by the above-mentioned.   A cast slab comprising the component composition according to any one of claims 1 and 3 to 6 is heated to 1100 ° C or higher, hot-rolled and pickled, and then 15% or more. After performing a cold rolling at a rolling rate of 1 ° C./sec after performing an annealing step of annealing the obtained cold-rolled steel sheet at a recrystallization temperature of −30 ° C. or lower and a recrystallization temperature of −60 ° C. or higher. As mentioned above, the manufacturing method of the high strength steel plate excellent in the shape freezing property characterized by performing the cooling process cooled to 480 degrees C or less with the average cooling rate of 200 degrees C / sec or less.   The high-strength steel sheet having excellent shape freezing property according to claim 8, wherein the annealing step is performed at a temperature range of not more than the maximum temperature and not less than the maximum temperature of -30 ° C for not less than 15 seconds and not more than 250 seconds. Production method.   The method for producing a high-strength steel sheet with excellent shape freezing property according to claim 8 or 9, wherein after the cooling step, hot dip galvanizing is performed and alloying treatment is performed.

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